Cell-substrate adhesion is generally considered to be a multistep process involving recognition of extracellular matrix components by cell surface receptors, followed by cytoskeletal rearrangements that lead to cell spreading (Grinnell, 1978; Hynes, 1981). Several extracellular matrix glycoproteins, such as fibronectin (Ruoslahti, et al., 1981b), laminin (Timpl, et al., 1979), vitronectin (Hayman, et al., 1983), and collagens have been shown to promote attachment of various cell types to tissue culture substrates (Kleinman, et al., 1981). The cell membrane receptors that recognize these matrix proteins, however, remain essentially unknown, although putative receptors for laminin (Lesot, et al., 1983; Malinoff and Wicha, 1983) and collagens (Chiang and Kang, 1982; Mollenhauer and yon der Mark, 1983) are currently being investigated.
A number of candidates for the role of a fibronectin receptor have been proposed. By photoaffinity labeling, it was shown that a 49 kd glycoprotein comes into close contact with substrate-bound fibronectin (Aplin, et al., 1981). Further support for the notion that the receptor is a protein comes from studies showing that treatment of cells with certain proteases abolishes the ability of cells to attach to fibronectin (Tarone, et al., 1982). Treatment with trypsin, however, at least in the presence of Ca.sup.++, leaves the receptor activity intact (Oppenheimer-Marks and Grinnell, 1984). Based upon the calcium-dependent stability to trypsin, Oppenheimer-Marks and Grinnell (1984) have proposed a 48 kd wheat germ agglutinin-binding glycoprotein as a potential fibronectin receptor.
It has also been suggested that heparan sulfate proteoglycans might be involved in cell attachment to fibronectin (Culp, et al., 1979; Laterra, et al., 1983). Indeed, photocross-linking experiments performed by Perkins, et al. (1979) showed that proteoglycans are associated with fibronectin at the cell surface.
A different type of cell surface component has been implicated in fibronectin-cell interactions by studies showing an inhibitory effect of di- and trisialogangliosides on the attachment of cells to fibronectin (Kleinman, et al., 1979). The inhibitory activity was found to reside in the carbohydrate moiety of the glycolipid. Antibodies that interfere with cell attachment have been described by a number of investigators, and the corresponding antigens have been found to be proteins with molecular weights ranging from 60 to 160 kd (Hsieh and Sueoka, 1980; Knudsen, et al., 1981; Neff, et al., 1982; Greve and Gottlieb, 1982; Oesch and Birchmeier, 1982), or specific gangliosides (Dippold, et al., 1984).
A large number of binding affinities are known to be present in the fibronectin molecule, such as for collagen (Engvall and Ruoslahti, 1977), fibrinogen and fibrin (Ruoslahti and Vaheri, 1975), proteoglycans (Stathakis and Mosesson, 1977), cell surfaces (Klebe, 1974; Pearlstein, 1976), and actin (Keski-Oja, et al., 1980), and there have been some studies of the interaction of cell surfaces with the cell attachment site. (Pierschbacher, et al., 1981). A large fibronectin fragment, that promotes cell attachment but lacks the other binding activities is also known, (Pierschbacher, et al., 1982, 1983; Pierschbacher and Ruoslahti, 1984a).
It has now been discovered that a 140 kd protein from detergent extracts of cells, when incorporated into liposomes, promotes their binding specifically to fibronectin-coated substrates via the Arg-Gly-Asp sequence in the fibronectin molecule.